It sounds like what you want is either MatchTagsPlugin by Eric Shulman (The TiddlyTools guy) or YourSearchPlugin by Udo Borkowski.
Both let you do full operator-enhanced tag searches but their syntax and other features differ.
MatchTagsPlugin lets you set up a new search field which displays its results in a new tiddler in a configurable format and will search only tags using a syntax like this:
excel AND (macro OR script) AND NOT virus
If the provided interfaces don't do it for you, take a look at what I did using InlineJavascriptPlugin.
YourSearchPlugin modifies the existing search field to support a richer query syntax (and, optionally, a new results popup) and you can specify that a given entry should only match tags by prefixing it like this:
#excel (#macro or #script) not #virus
You could also poke around on TiddlyHub to see if anything else might meet your needs.
Re your point 2
You are heading in the same direction my company has gone. We have the equivalent of your 'Central System' as a Username, and have created three folders on its Drive - WORK, REFERENCE, EXTERNAL USERS - and shared them with our Sysadmin. Sysadmin shared WORK with Users on an Edit basis and our users built a sub-folder structure for our company to hold all our working files. Sysadmin shared REFERENCE with most Users on Read basis but with some Users granted Edit rights so they could create and maintain the folder/file structure for policy documents, manuals etc.
WORK is shared Edit and REFERENCE is shared Read to new starters as part of the User setup process and new Users immediately acquire inherited rights to all the contents of these folders.
Leavers, we change the User password immediately, use administration advanced tools to transfer ownership to their manager/colleague for any files/folders they may have created and then unshare them from WORK and REFERENCE.
We have not made WORK or REFERENCE to be the Owner of the sub-folders and documents which they contain because it would require that only someone logged into Central System would be able to delete them. We simply use our own version of Central System to provide a unified data structure for our Users. The Users each retain the 5gb of non-Gdocs storage space which is shared with other Users through WORK and REFERENCE.
We collaborate with external organisations and use EXTERNAL USERS as a portal. For example, our Auditors want access to our some of the data on our system. We created a sub-folder AUDITACCESS in the EXTERNAL ACCESS folder and granted our auditors' gmail accounts Read access to AUDITACCESS. We then use the Organise function to give them a View towards the files and folders which they need to access (but remember to hold the CTRL key down when you use the Organise function). This allows us to un-Organise files and folders, if necessary, since they are all listed in the AUDITACCESS folder and to un-share AUDITACCESS with our auditors email accounts, if necessary.
Hope this will give you a few ideas for organising your own system.
Best Answer
If you haven't found a solution, I would suggest looking to a service like Dropbox for hosting the wiki. Google posted that they are deprecating web hosting support in Google Drive come August. A good alternate would be Dropbox. I briefly skimmed Dropbox's help section. I would say you should be able to share your blog with just your buddies, giving them read and/or write access.